to ensure she doesn’t break anything or end up where she shouldn’t…
So she uses an operating system that’s near to 2 decades old OS with years of live without any security patches whatsoever? I would say that’s rather more dangerous for her, than using an up to date operating system.
Just install an up to date os of your choice (be it Linux (Mint) or Windows) and create her an account without admin privileges and some reliable antivirus (especially on windows). If you want to be really safe you can filter her network traffic. There are countless tools for this. One of the easier ones to use is NextDNS even though it’s possible there are even better solutions.